Equilibration
Biting forces in this simulation model (of a common malocclusion) are completely borne by the second molars, placing heavy forces on those teeth. These patients cannot even get the other teeth to touch. This makes chewing very inefficient, and can lead to dietary modification and digestive problems. It also makes the second molars high risk for fractures.
The term equilibration literally means “to establish balance or equality.” With regard to the jaw joints, jaw muscles and teeth (the stomatognathic system), it means adjusting the way the teeth mesh such that when the jaw muscles are at rest, and the jaw joints are in a stable, neutral position, the teeth are engaging uniformly with equal contacts between the upper and lower teeth.
The importance of equal forces on the teeth is intuitively easy to understand. If only a few teeth engage when a person bites together, those teeth will endure much more force than they are structurally able to withstand. They will wear excessively (a condition called attrition), chip, crack, or the patient will (often subconsciously) continuously move their jaw in an attempt to locate a position where the forces are more in balance. Unfortunately, repositioning the jaws to provide balanced force distribution on the teeth may not be possible, due to crowding or misalignment of the teeth. The problem of poorly distributed bite forces on the teeth is magnified if teeth are lost.
Casts of the patient’s teeth have been constructed from green dental stone, and mounted on a jaw simulation tool called an articulator. The teeth have been painted red to clearly show where adjustments are made and give an idea of how much tooth structure would need to be removed.
The patient’s jaw muscles may become fatigued (myalgia) by continuously trying to achieve a position of balanced force distribution on the teeth. The muscles may also become painful. The patient may chronically grind (brux) their teeth until they wear the teeth into a position of more equal force distribution. The jaw joints themselves may be the weak link, and develop popping, clicking, grating (crepitus) or pain from the unequal balance of forces. Collectively, these symptoms represent various stages of temporomandibular joint (TMJ) dysfunction, or TMD.
To understand the process of equilibration, it is important to understand a little bit about how the temporomandibular joints (i.e. the jaw joints) work. They are not pure hinge joints like knees or elbows. Instead, only the first 15 degrees or so of opening is rotational (hinge). With further opening, the lower jaw begins to slide along the skull base until it reaches the maximum opening. It is the only joint like this in the body.
The areas where green stone is showing through the red paint show the adjustments that were made to the casts to achieve balanced forces. A well-equilibrated bite relationship was able to be established in this patient with surprisingly little removal of tooth structure.
In order for this type of motion to be possible, the lower jaw (mandible) must be free to move in all three dimensions (forwards, backwards, and side-to-side). It does that by contraction of the different jaw muscles in a 3-way “tug-of-war”. The distance the jaw can move in any direction is limited by the length of the muscles that move it the opposite direction. The stable position is one in which all of the muscles are neutral and not contracting (pulling). If the teeth do not touch evenly with the muscles in that position of balance, problems can occur.
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